Radiosurgery is used to treat tumors and other lesions by delivering a prescribed high dose of high-energy radiation to the target area while minimizing radiation exposure to the surrounding tissue. In radiosurgery, precisely focused beams of radiation (e.g. very intense x-ray beams) are delivered to a target region in order to destroy tumors or to perform other types of treatment. The goal is to apply a lethal amount of radiation to one or more tumors, without damaging the surrounding healthy tissue. Radiosurgery therefore calls for an ability to accurately target a tumor, so as to deliver high doses of radiation in such a way as to cause only the tumor to receive the desired dose, while avoiding critical structures such as the spinal cord.
Conventional radiosurgery uses a rigid and invasive stereotactic frame to immobilize the patient prior to diagnostic CT or MRI scanning. The treatment planning is then conducted from the diagnostic images. The treatment planning software determines the number, intensity, and direction of the radiosurgical beams that should be cross-fired at the target, in order to ensure that a sufficient dose is administered throughout the tumor so as to destroy it, without damaging adjacent healthy tissue. Radiation treatment is typically accomplished on the same day treatment planning takes place. Immobilization of patient is necessary in order to maintain the spatial relationship between the target and the radiation source to ensure accurate dose delivery. The frame is fixed on the patient during the whole treatment process.
Image-guided radiosurgery eliminates the use of invasive frame fixation during treatment, by frequently and quasi-continuously correcting patient position or aligning radiation beam with the patient target. To correct patient position or align radiation beam, the patient pose during treatment needs to be detected. This is accomplished by registering the X-ray image acquired at the treatment time with the diagnostic 3D scan data (e.g., CT, MRI, or PET scan data) obtained pre-operatively at the time of treatment planning. The positions of the target are defined by physicians at the time of treatment planning, using the diagnostic 3D scan. The 3D scan data are used as a reference, in order to determine the patient position change during treatment. Typically, digitally reconstructed radiographs (DRRs) are generated from the 3D scan data, and are used as 2D reference images. Similarity measures are used to compare the image intensities in the x-ray images and the DRR images, in order to determine the change in the position of the patient and the treatment target. In the field of medical image registration, this problem is categorized as a 2D/3D registration.
Image-guided radiosurgery requires precise and fast positioning of the target at the treatment time. In practice, the accuracy should be below 1 mm, and the computation time should be on the order of a few seconds. Unfortunately, it is difficult to meet both requirements simultaneously, because of a number of reasons. First, the two different image modalities (CT and x-ray) are characterized by different spatial resolution and image quality. X-ray image resolution and quality are generally superior to the resolution and quality of DRR images, which are only synthesized projection images. Second, DRR generation relies on a proper attenuation model. Because attenuation is proportional to mass intensity, the exact relationship between mass intensity and CT image intensity needs to be known for an accurate modeling. It is difficult to establish this relationship, however, so the linear attenuation model is often used. The skeletal structures in DRR images cannot be reconstructed very well when the linear model is used. Finally, x-ray images usually have a large image size (512×512). It is desirable to use full resolution images, for better registration accuracy. However, the full resolution of the x-ray images is rarely used, because of the extremely slow computation time that results from such use.
The methods used in the 2D/3D registration can be categorized into two types. The first type of methods is based on image features. The image features could be anatomical edges, for example, or segmented objects. The registration accuracy depends on the accuracy of edge detection or object segmentation. The main advantage of this type of method is the high computation speed. Because the full information content of the image is not used, however, accuracy is sacrificed. The second type of method is based on image intensity content. The original images are used for registration. Usually, a good accuracy can be achieved. Because a long time computation is required, however, image intensity based methods are not practical for radiosurgery, or for clinical practice in general.
U.S. Pat. No. 5,901,199 by Murphy et al. (the “Murphy patent”) describes a high-speed inter-modality image registration via iterative feature matching. The Murphy patent is a feature-based method. Prior to treatment, extraction and segmentation of silhouettes of the patient's skull are performed in order to make a feature mask. A set of DRR images are generated from the 3D CT data and are then masked, in order to isolate key pixels that are associated with anatomical edge features. The masked image contains only 5%-10% of the total image pixels. During treatment, the acquired x-ray images are similarly masked. The registration is conducted on the masked DRRs and the masked X-ray images. The registration process is completed in a few seconds. However, the accuracy and stability of the estimates are not sufficient to meet the sub-mm precision that is required in radiosurgery applications.
For these reasons, there is a need for a method and system in image-guided radiosurgery for tracking the position of the treatment target, throughout the radiosurgical treatment procedure, with a computation time that is fast enough for purposes of radiosurgery, while at the same time maintaining sufficient accuracy and stability.